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Michael Love & Associates (MLA) served as the fish passage engineers for design of six fish passage restoration projects for Marin County road-stream crossings. Working with the Marin County Department of Public Works, MLA developed solutions that met fish passage, stream protection, and transportation objectives and constraints. Projects included stream simulation crossings, nature-like roughened channels in new crossings and below an existing culvert, and a bypass switchback pool and weir fishway below a perched box culvert outlet.

Stream Simulation Road-Stream Crossings

The two stream simulation crossings were designed to maintain self-sustaining stream channels within the culverts that mimic the natural channel morphology. They consist of open-bottom arch culverts that span the bankfull channel and sized to provide 100-year flood conveyance with freeboard for debris passage. MLA conducted geomorphic assessments to establish the current stable channel profile and the profile’s potential range of vertical self-adjustment during the service life of the crossing. This, along with a scour analysis following FHWA methods, established the depth of the footings for the arch culverts. Rock streambanks were constructed within the culverts to create flow diversity along the channel margins similar to the natural channel, and eroded and over-widened stream banks at the culvert outlet were restored.

Nature-Like Roughened Channels

MLA designed four-percent sloped nature-like roughened channels constructed through two new open-bottom arch culverts. Both crossings were located where the downstream channel had incised, leaving the pre-project culverts perched and creating a leap barrier for native salmon and steelhead trout. These roughened channels with designed as rigid-bed channels to protect the stability and habitat of the upstream channel from incision while providing passage for native fish and other aquatic organisms.
A third roughened channel was designed by MLA for restoring an incised channel downstream of a perched box culvert. This project involved constructing a 120-foot long rock chute to steepen the channel profile and allow fish to swim into the culvert, thus eliminating a leaping barrier.

For all three nature-like roughened channels, MLA sized the engineered streambed material (ESM) based on USACE rock sizing equations and California Dept. of Fish and Game (2009) recommended ESM gradations. The construction drawings included a rock-placement plan to create the desired steep-channel morphology. MLA also calculated fish passage design flows and fish passage hydraulics, including water depths, velocities, and energy dissipation factor (EDF, a metric of turbulence).

Bypass Pool and Weir Fishway

MLA designed a bypass switchback pool and weir style fishway for providing fish passage at a 7.5-foot perched box culvert outlet. Because the culvert was located at the stream’s confluence with the main-stem channel, there was limited space to construct a fishway. After conducting an alternatives analysis, the County selected the compact switchback fishway to provide upstream passage for adult and juvenile salmonids. The “bypass fishway” is designed to convey a portion of the total streamflow, allowing high flows and debris to bypass the fishway. The fishway is tucked into the existing streambank and contains a switchback to place the entrance (downstream end) close to the barrier (culvert outlet). This allows fish that approach the barrier to easily locate the fishway entrance.

MLA sized the pools based on National Marine Fisheries Service (NMFS) criteria for depth and EDF. Weirs were shaped to create desirable hydraulics for leaping by juvenile salmonids, and designed with stoplogs that may be removed to flush accumulated sediments and drain pools in the dry season to prevent fish stranding. A service path was included in the design to allow maintenance workers easy access to the entire fishway for removing any debris accumulations. MLA also designed angled fish baffles for the existing culvert to slow water velocities and increase water depths, such that fish existing the fishway are able to swim through the culvert at all fish passage design flows.

To-date, five projects has been constructed and the sixth is awaiting implementation funding. Working closely with the regulatory agencies and funders allowed most of these these projects to be constructed shortly after the designs were completed.